Orthopedic implant materials must combine high strength, corrosion resistance and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient of the implant is relatively young because it is desirable that the implant function for the complete lifetime of a patient. Because certain metal alloys have the required mechanical strength and biocompatibility, they are ideal candidates for the fabrication of prostheses. These alloys include 316L stainless steel, chrome-cobalt-molybdenum alloys and, more recently, titanium alloys which have proven to be the most suitable materials for the fabrication of load-bearing prostheses.
It has also been found that metal prostheses are not completely inert in the body. Body fluids act upon the metals causing them to slowly corrode by an ionizing process that thereby releases metal ions into the body. Metal ion release from the prosthesis is also related to the rate of wear of load bearing surfaces because the passive oxide film, which is formed on the surface, is constantly removed. The repassivation process constantly releases metal ions during the ionizing process. Furthermore, the presence of third-body wear (cement or bone debris) accelerates this process and microfretted metal particles increase friction.
The excellent corrosion resistance of zirconium has been known for many years. Zirconium displays excellent corrosion resistance in many aqueous and non-aqueous media and for this reason has seen an increased use in the chemical process industry and in medical applications. A limitation to the wide application of zirconium in these areas is its relatively low resistance to abrasion and its tendency to gall. This relatively low resistance to abrasion and the tendency to gall is also demonstrated in zirconium alloys.
U.S. Pat. No. 2,987,352 to Watson first disclosed a method of producing zirconium bearings with a specific form of oxidized zirconium as a surface layer. The method of Watson was refined by Haygarth (U.S. Pat. No. 4,671,824) resulting in improved abrasion resistance and better dimensional control of the oxidized product. The U.S. Pat. Nos. of Davidson (5,037,438; 5,152,794; 5,180,394; 5,370,694; 5,372,660; 5,496,359; and 5,549,667) demonstrated the many advantages that are realized through the use of the specific form of oxidized zirconium on zirconium and zirconium alloy substrates in prosthetic devices. These include increased strength, low friction and high wear resistance. U.S. Pat. No. 5,037,438 to Davidson first disclosed a method of producing zirconium alloy prostheses with an oxidized zirconium surface. The work of Watson and Davidson teach a specific form of oxidized zirconium which possesses all of the advantages of ceramic materials while maintaining the strength of metallic surfaces. The oxidation is characterized by the diffusion of free oxygen into the surface of the metal; the resulting oxide layer is characterized by the diffusion of free oxygen into the surface of the metal. The resulting “diffusion hardened” materials possess a unique combination of the advantageous properties of a ceramic and a metal, simultaneously minimizing the disadvantages of these materials. All of the U.S. patents cited above to Davidson, Watson, and Haygarth are incorporated by reference as though fully set forth herein. While the early work of Davidson focused on pure zirconium and alloys of zirconium in which zirconium was the predominant metal, later work has shown that this is not necessary in order to form the desired diffusion hardened oxide. For instance, an alloy of 74 wt % titanium, 13 wt % niobium and 13 wt % zirconium (“Ti-13-13”) will form the diffusion hardened oxidation layer used herein. Ti-13-13 is taught in U.S. Pat. No. 5,169,567 to Davidson et al.
Another important performance criterion for medical implants is the degree of fixation stability. This is typically accomplished through ingrowth of surrounding tissue into the implant and its ability to become firmly anchored to other components such as bone cement with a large shear strength. A typical hip joint prosthesis includes a stem fixated into the femur, a femoral head, and an acetabular cup against which the femoral head articulates. A typical knee joint prosthesis has a femoral and tibial component, both of which are fixated to the respective bones. This is the stability with which the implant is anchored in place. This fixation could be to either bone or other tissue, or may consist, at least in part, of materials, such as bone cement, etc. The fixation stability of the prostheses of Davidson was realized in their use of porous metal beads or wire mesh coatings the promoted bone in-growth and increased surface area for adhesion to other materials. These techniques are taught in U.S. Pat. No. 5,037,438 and other patents of Davidson, and when combined with the advantages of oxidized zirconium, represented an improvement in performance of medical implants in numerous areas. Nevertheless, continued improvement in the fixation stability of such implants is desirable.
A principle goal in the field of prosthetic implants is the lengthening of the useful life of the implant such as to avoid or minimize the need for surgical revision or replacement. A delay or complete prevention of failure of an implant is desirable. The causes of implant failure are numerous. It is believed that the failures are attributable to the body's rejection of bone cement. It is also believed that rejection of bone cement is not the primary problem, but rather that bone cement is not a proper structural component for use as part of a joint implant because of its physical properties.
Specifically, natural bone has a modulus of elasticity of up to about 4×106 p.s.i. The metals used for implants generally have a modulus of elasticity on the order of 15-35×106 p.s.i. Polymethylmethacrylate (PMMA) cement, on the other hand, has a modulus of elasticity on the order of 0.3-0.5×106 p.s.i. The stiffness of PMMA cement is therefore less than either the metal prosthesis or the surrounding bone. Cement has lower mechanical properties strength and fatigue strength properties than does metal or bone. These comparative physical properties are thought to be the source of failure of hip and knee prostheses implanted using bone cement.
Prostheses may also be implanted without cement. These devices achieve fixation by in-growth of bone or tissue into the prosthesis or by wedging the prosthesis into bone. The devices may also include surface features which enhance ingrowth with fibrous tissue or bone. The surface features may be applied by deposition or spraying techniques.
It is generally understood that surface roughening results in increased surface area which typically leads to better adhesion for the fixation of two surfaces. Although a smooth surface minimizes the stresses within the implant, it also minimizes the total surface area. This decreased surface area significantly reduces the strength of the attachment of the implant to the bone and tissue, which is largely dependent upon the mechanical interaction of the implant and the tissue. This mechanical interaction is of two forms. One is a form of interlocking to the extent the tissue grows behind or around a part of the implant. The other is frictional, wherein the tissue grows into intimate approximation with the surface and results in a relatively tight frictional fit.
Wagner et al. have demonstrated a method in U.S. Pat. No. 5,922,029 (and the resulting product in U.S. Pat. No. 6,193,762) using an electrochemical etching techniques to create attachment surfaces having random irregular patterns that promote bone tissue ingrowth and also to facilitate joining of the surface to a second material. Wagner et al. teach analogous methods (U.S. Pat. No. 5,258,098) and medical implant products (U.S. Pat. No. 5,507,815) in which the etching methodology used is purely chemical. Although the techniques of Wagner et al. represent one potential source of methods for surface texture modification it is expected that any other surface texture modification techniques would be similarly useful in aiding fixation. For example, the teachings of Frey (U.S. Pat. No. 4,272,855), Van Kampen (U.S. Pat. No. 4,673,409, Sump (U.S. Pat. No. 4,644,942), and Noiles (U.S. Pat. No. 4,865,603), among others, can be combined with in situ diffusion hardened surface oxidation of Davidson to produce a prosthesis surface having the superior attributes of surface oxidation as well as the stabilization and in-growth enhancement benefits accruing from macroscopic texture modification.
There exists a need for a method to produce medical implants having improved fixation while preserving or improving the advancements realized through the use of oxidized zirconium. This improved stability is needed both with respect to the interface between the implant and bone and surrounding tissue as well as in the interface between the implant and other material such as bone cement. This should be accomplished while simultaneously preserving the advantages which inure through the use of in situ oxidized, diffusion hardened surfaces such as oxidized zirconium.